The typical prior art exhaust muffler comprises a plurality of parallel tubes supported by an array of transverse baffles. The tubes and baffles are disposed in a tubular shell formed by one or more sheets of metal. The shell typically is of oval or circular cross section and is aligned parallel to the tubes therein. The shell abuts the similarly shaped baffles to define chambers within the prior art muffler. Heads are mechanically attached or welded to the opposed ends of the shell and tubular nipples extend through the heads to provide communication with the tubes and chambers in the prior art muffler.
The tubular components of the prior art muffler define a carefully engineered flow path for exhaust gases. For example, many prior art mufflers include an inlet tube that extends into a reversing chamber defined by the baffles and the shell, while a return tube extends from the same reversing chamber to enable the exhaust gases to undergo a 180.degree. change in direction. In many instances, two or more tubes extending through a chamber are perforated. Thus, while a primary flow of exhaust gas travels axially through the tubes, a secondary generally radially directed flow is established out the perforations of one tube, through the chamber and into the perforations of another tube. The proportional distribution between the axial flow through the tube and the radial flow through the perforations depends on the flow rates of the exhaust gases, the diameters of the respective tubes, and the total area of the perforations in the respective tubes. Varying any of these parameters can significantly affect the noise attenuation and flow characteristics of the prior art muffler.
In many situations, the above described carefully engineered tuning leaves one or more residual frequencies that are not properly attenuated. These residual frequencies typically are attenuated by the combination of tuning tubes and an enclosed resonating chamber. One end of the tuning tube may communicate with a reversing chamber in the muffler, while the opposed end of the tuning tube communicates with the resonating chamber. The diameter and legnth of the tuning tube and the volume of the resonating chamber are carefully engineered to attenuate one of the residual frequencies.
In certain instances, the tuning tube and resonating chamber cancel to some degree the initially observed residual frequency, but create a second and usually closely related residual frequency. Muffler designers have discovered that this shift of residual frequencies can often be eliminated by providing an aperture in one of the baffles defining the low frequency resonating chamber. These apertures cause the resonating chamber to attenuate a broader range of frequencies than the specific frequency dictated by the dimensions of the tuning tube and the resonating chamber.
The above described prior art exhaust muffler requires a substantial number of separate parts which require a corresponding high number of manufacturing steps, many of which are not well suited to automation. As a result, most prior art muffler manufacturing has been labor intensive.
Attempts have been made to manufacture exhaust mufflers from two shells stamp formed to define a circuitous path through which the exhaust gases must travel. These types of prior art stamp formed mufflers are shown in U.S. Pat. No. 2,484,827 which issued to Harley and U.S. Pat. No. 3,638,756 which issued to Thiele.
Certain other stamp formed mufflers have a plurality of plates, including internal plates stamp formed to define perforated tubular passages and external shells stamp formed to surround and enclose the perforated tubular passages. For example, British Pat. No. 632,013, which issued in 1949, shows internal plates stamp formed to define a circuitous array of perforated tubes, and a pair of external shells stamp formed to define an enclosure around the internal plates. British Pat. No. 1,012,463, which issued in 1965, shows a similar muffler; however, certain portions of the internal plates are stamp formed to define hinged flaps which are rotated out of the plane of the plate to define internal baffles. Additionally, the internal plates of the muffler shown in British Pat. No. 1,012,463 shows a plurality of stamp formed perforated tubes and stamp formed apertures in proximity to the tubes. U.S. Pat. No. 4,132,286 which issued to Hasui et al on Aug. 25, 1977 shows a stamp formed muffler very similar to the muffler shown in British Pat. No. 1,012,463. However, U.S. Pat. No. 4,132,286 further shows a single tube having an array of apertures or shunts at an upstream location and having a stamp formed taper to reduce the diameter at a downstream location. The relative sizes of the upstream shunts and the downstream reduced diameter portions are selected to vary the relative flows through the upstream shunts and the downstream apertures. These prior art stamp formed mufflers have attempted to model the outer shell mufflers, and thus included tubular portions spaced inwardly from the external shells.
Until very recently, stamp formed mufflers did not provide the complex flow patterns and the carefully engineered tuning that had been achieved with the prior art wrapped outer shell mufflers having separate internal tubular components and baffles. However, U. S. patent application Ser. No. 934,642, filed Nov. 25, 1986, which is entitled "STAMP FORMED MUFFLER" by Jon Harwood and which is assigned to the assignee of the subject application shows a muffler having all of the desirable attributes of stamp forming while still achieving the precisely engineered tuning. The muffler shown in application Ser. No. 934,642 includes at least one expansion chamber in communication with perforated tubes, and at least one low frequency resonating chamber in communication with a tuning tube.
Despite the many advantages of the muffler shown in Ser. No. 934,642, it has been found that certain mufflers having a plurality of closely spaced expansion chambers and/or low frequency resonating chambers connected by stamp formed tubes could often require excessive deformations of the metal. With certain types of metals, such as 0.034 inch thick stainless steel, the extreme deformations that were believed to be required to create expansion chambers and resonating chambers would result in unacceptably high reject rates. The reject rates were primarily caused by ruptures of the metal during the stamp forming operation, and typically occurred where the tubular portions extended between closely spaced chambers. Additionally, despite the many advantages of the muffler shown in Ser. No. 934,642, it was considered desirable to improve even further upon the tuning capabilities of stamp formed mufflers, and to enhance the strength and acoustical insulation of stamp formed mufflers.
In view of the above, it is an object of the subject invention to provide a muffler that can be manufactured with high reliability and quality.
Another object of the subject invention is to provide a stamp formed muffler that reduces the amount of metal deformation required to create separate expansion chambers and/or low frequency resonating chambers.
An additional object of the subject invention is to provide a muffler having at least one low frequency resonating chamber that is adapted to substantially soften a narrow range of objectionable low frequency noise.
A further object of the subject invention is to provide a muffler that achieve a carefully controlled cross flow of exhaust gases between two or more tubular members.
Still another object of the subject invention is to provide a formed muffler of enhanced strength.